Earth-mass planet resembles a mini-Neptune

Exoplanet 200 light-years away made mostly of gas, not rock

GASSY EARTH The planet KOI-314c, at right in this illustration, has about the same mass as Earth but is 60 percent larger. Another planet around the star (top) has the same diameter as KOI-314c but is made of rock, so it’s much heavier.

OXON HILL, Md.— A newly discovered exoplanet weighs about the same as Earth — but that’s where the similarities end. Its discovery, reported January 6 at the American Astronomical Society meeting and online at arXiv.org, illustrates the complexities of classifying planets outside the solar system.

The planet, KOI-314c, orbits a small, cool star 200 light-years away in the constellation Lyra. While the planet’s mass is similar to Earth’s, its diameter is 60 percent larger, said David Kipping, a Harvard-Smithsonian Center for Astrophysics astronomer. That yields a density consistent with a world composed largely of gas. It’s the first time astronomers have been able to compare the composition of an Earth-mass planet to Earth.

About three-quarters of the planets identified by NASA’s Kepler space telescope, including KOI-314c, are larger than Earth but smaller than Neptune. At nearly four times the size of Earth, Neptune is the next-smallest planet in the solar system. So astronomers lack nearby planets to compare with these larger-than-Earth ones.

Results also announced at the AAS meeting indicate that most worlds smaller than twice Earth’s size are more like big, rocky Earths than like miniature, gassy Neptunes. But KOI-314c shows that “there’s a blurry divide,” Kipping said.